Throughout this application, various publications are referenced within parentheses. Disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains. Full bibliographic citations for the references may be found listed immediately preceding the claims.
In plants, the developmental control of morphogenesis entails the coordination of cell growth, cell division and cell differentiation, and undoubtedly, organ size is one of the most obvious reflections of this coordination (Mizukami, 2001). Organ size is determined both by cell size and cell number. Genetic studies have revealed that differences in cell size or cell polar elongation apparently contribute to the size difference of plant organs (Kim et al., 1998; Kondorosi et al., 2000; Kim et al., 2002). On the other hand, larger organs tend to contain more cells than smaller ones, implying that cell division plays a fundamental role in organ size determination during organogenesis. Some mutants with altered organ size, such as struwwelpeter (swp) and phantastica (phan) (Waites et al., 1998; Autran et al., 2002), indeed show a decreased or increased cell number in their organs. However, there are cases in which alteration of cell proliferation is not always correlated with changes in organ size. For example, expression of a dominant-negative Arabidopsis CDKA in tobacco results in the almost normal size of leaves with fewer but larger cells (Hemerly et al., 1995). In Arabidopsis, over-expression of CycD3;1,a G1 cyclin gene, fails to increase organ size, leading to a disturbed organogenesis with numerous small, incompletely differentiated cells (Riou-Khamlichi et al., 1999; Dewitte et al., 2003). Similar data were obtained from over-expression of E2Fa and DPa, two transcriptional factors that play a role in activating cell division gene. Co-expression of E2Fa and DPa in Arabidopsis causes extra cell division but early arrested growth of plants (De Veylder et al., 2002). These observations suggest the existence of an intrinsic mechanism to coordinate cell proliferation and growth, by which the organ development is strictly controlled (Beemster et al., 2003).
The aerial organs in plant come from the promordia initiated from apical and lateral meristems. Significant changes in morphology and size of organs occur when the specification or growth of these meristems or primordia is disorganized or interrupted. A considerable number of genes involved in this developmental process has been identified and characterized, such as WUSCHEL (WUS), CLAVATAs (CLVs), and SHOOT MERISTEMLESS (STM) (Meyerowitz, 1997; Golz and Hudson, 2002). Nevertheless, lateral organ growth appears to rely on the interactive and durable division of cells within organ or organ meristems (Mizukami, 2001). Although cells in plant organs remain theoretically in an indeterminate dividing state or even a differentiated cell in plant can revert to a stem cell (Weigel and Jugens, 2002), the determinate organ growth destines these cells to stop dividing as an organ develops. Thus, cell meristematic competence appears to be critical to the cell proliferation within organ and thereby organ size (Mizukami, 2001). Recent studies on Arabidopsis Aintegumenta (ANT) apparently strengthen this view. ANT seems to function as a coordinator of cell proliferation and lateral organ development. Loss-of-function of ANT reduces the size of leaf and floral organs (Elliott et al., 1996; Klucher et al., 1996; Mizukami and Fischer, 2000), whereas ectopic expression of ANT increases the size of leaf, inflorescence stem and floral organs. These alterations result mainly from changes in total cell number (Krizek, 1999; Mizukami and Fischer, 2000). Further examination reveals that ANT does not affect the growth rate but regulates the extent of organ growth by maintaining the meristematic competence of organ cells, thereby defining intrinsic organ size (Mizukami and Fischer, 2000). At present, the molecular nature of meristematic competence remains unclear (Weigel and Jugens, 2002). In addition, given their sessile and light-dependent life style, organ size in plant is also greatly influenced by environmental and developmental signals, including light, nutrients, and especially plant hormones. Nevertheless, how these signals affect organ development is poorly understood.
The plant hormone auxin plays an essential role in a wide variety of plant growth and developmental process, such as shoot and lateral root formation, apical dominance, tropism and senescence (Davies, 1995). Recent genetic and biochemical analyses have suggested that the ubiquitination-regulated proteolysis is central to several aspects of auxin response (Gray et al., 1999; Gray et al., 2001; Dharmasiri and Estelle, 2002; Kepinski and Leyser, 2002; Leyser, 2002). As a model system, some advances in how auxin promotes lateral root formation have been reported recently (Xie et al., 2000; Casimiro et al., 2001; Xie et al., 2002). However, little is known as to how auxin regulates development of aerial parts of plant. At the cellular level, auxin acts as a signal for cell division, expansion and differentiation (Leyser, 2001), and some lines of evidence at the whole plant level indicate that auxin plays a role in organ cell proliferation as well as organ size (Lincoln et al., 1990; Ecker, 1995). For example, mutation of Arabidopsis REVOLUTA (REV)/INTERFASCICULAR FIBERLESS1 (IFL1) prolongs the growth and cell proliferation, resulting in larger leaves, flowers and thicker inflorescent stems (Talbert et al., 1995; Zhong and Ye, 1999). The REV/IFL1 is involved in auxin polar transport, shoot secondary meristem formation and differentiation of interfascicular fibre cell (Zhong and Ye, 2001), suggesting that polar auxin flow may also influence organ development. By contrast, the auxin resistant 1 (axr1) mutant has obviously smaller leaves, inflorescence stems and floral organs, and anatomic examination shows that the reduced size of leaf and stem is caused by a decrease in cell number rather than cell size (Lincoln et al., 1990). Although these observations suggest that AXR1 might be involved in auxin-dependent cell proliferation during development, there is as yet no molecular data to support this claim.
There remains a need in the art, therefore, for a greater understanding of the mechanisms of plant organ development, for methods of regulating this development, and for plants and plant cells in which such development can be regulated.